The objective of the research is to develop a P. carinii-free animal model for use in the study of transmission modes and habitat of the organism. Initial studies evaluated the efficacy of trimethoprim-sulfamethoxazole (TMP-SMZ) in the eradication of P. carinii from conventional Sprague-Dawley rats. Animals treated with TMP-SMZ for 2, 4 or 6 weeks were then placed in isolator cages and immunosuppressed with prednisone. Upon sacrifice after 3 months of corticosteroid therapy, all of the untreated control animals and 80 percent, 90 percent and 100 percent of those treated for 2, 4 and 6 weeks respectively, with TMP-SMZ had P. carinii infection in the lungs. Examination of 400 peripheral blood smears from these animals failed to reveal a hematogenous phase of P. carinii. The study shows that TMP-SMZ does not eradicate P. carinii from latently infected animals; that the incidence of naturally acquired P. carinii infection is remarkable high in the Sprague-Dawley rat and no hematogenous phase of the organism can be identified.